Production of polypropylene having improved properties

ABSTRACT

A process for producing polypropylene having increased melt strength, the process comprising irradiating polypropylene which has been polymerised using a Ziegler-Natta catalyst with an electron beam having an energy of at least 5 MeV and a radiation dose of at least 10 kGray and mechanically processing the irradiated polypropylene to form long chain branches on the polypropylene molecules, whereby the polypropylene has a melt flow index (MFI) of at least 25 dg/min.

The present invention relates to a method for the production ofpolypropylene, having improved properties, in particular melt strengthand high melt index whereby the polypropylene is suitable for theproduction of fibres. In particular, the present invention relates to aprocess for the production of polypropylene having improved propertiesby irradiating polypropylene with a high energy electron beam.

Polypropylene resin is used in a variety of different applications.However, polypropylene resin suffers from the problem of having a lowmelt strength at high melt index, which restricts the use ofpolypropylene in a number of applications because the polypropylene isdifficult to process, particularly in the production of fibres wherehigh melt index and sufficient melt strength are required. It is knownin the art to increase the melt strength of polypropylene, for exampleby irradiating the polypropylene with an electron beam. It is known thatelectron beam irradiation significantly modifies the structure of apolypropylene molecule. The irradiation of polypropylene results inchain scission and grafting (or branching) which can occursimultaneously. Up to a certain level of irradiation dose, it ispossible to produce from a linear polypropylene molecule having beenproduced using a Ziegler-Natta catalyst, a modified polymer moleculehaving free-end long branches, otherwise known as long chain branching.It is known that such long chain branching drastically modifies therheological behaviour of the polypropylene, for example theirelongational and shear viscosity.

EP-A-0678527 discloses a process for producing a modified polypropylenein which polypropylene and a cross-linking agent mixture are irradiatedwith ionising radiation so as to give an absorbed dosage of 1 to 20 kGy,with subsequent heat-treating of the resultant material. In Example 1 itis disclosed that the irradiation conditions have an accelerated voltageof 2 MW and an electric current of 1.0 mA.

WO-A-97/08216 discloses a method for producing diene-modified propylenepolymers which are irradiated. It is disclosed that the irradiation ispreferably carried out using E-beam or γ radiation at a dose of about 1to about 20 Mrad for a few seconds. It is disclosed that polypropylenemade be modified with a diene and then irradiated to cause chainextension.

EP-A-0634441 discloses a process for making a high melt strengthpropylene polymer by high energy radiation. The dose range is disclosedas being from 1 to 10,000 Mrads per minute and it is disclosed that theionising radiation should have sufficient energy to penetrate to theextent desired in the mass of linear, propylene polymer material beingradiated. There is also disclosed the use of an accelerating potential(for an electron generator) of 500 to 4000 kV. Following the irradiationstep the irradiated material is heated.

EP-A-0190889 discloses a process similar to that of EP-A-0634441 in thatit is disclosed that the accelerating potential of an electron generatormay be from 500 to 4000 kV.

EP-A-0799839 also has a similar disclosure to EP-A-0634441 and disclosesthe use of an electron generator having accelerating potential of 500 to4000 kV.

EP-A-0451804 discloses a method of increasing the molecular weight ofsyndiotactic polypropylene by irradiation in the absense of oxygen. Thisspecification does not disclose any energy range for the irradiation.The dose of the irradiation may be from 0.1 to 50 Mrad. Afterirradiation, the polypropylene may be heated.

EP-A-0351866 has a yet further similar disclosure to EP-A-0634441 anddiscloses the use of an electron generator having an acceleratingpotential of 500 to 4000 kV.

U.S. Pat. No. 5,554,668 discloses a process for irradiatingpolypropylene to increase the melt strength thereof. An increase in themelt strength is achieved by decreasing the melt flow rate, otherwiseknown as the melt index. It is disclosed that a linear propylene polymermaterial is irradiated with high energy ionising radiation, preferablyan electron beam, at a dose rate in the range of from about 1 to 1×10⁴Mrads per minute for a period of time sufficient for a substantialamount of chain scission of the linear, propylene polymer molecule tooccur but insufficient to cause gelation of the material. Thereafter,the material is maintained for a period of time sufficient for asignificant amount of long chain branches to form. Finally, the materialis treated to deactivate substantially all free radicals present in theirradiated material. It is disclosed that for an electron beam, theelectrons are beamed from an electron generator having an acceleratingpotential (i.e. an energy) of from 500 to 4000 kV. Typically, thepolypropylene material to be irradiated is in particulate form and isconveyed on a conveyor belt beneath an electron beam generator whichcontinuously irradiates the polypropylene particles as they aretranslated thereunder by the conveyor belt. The resultant polyethylenehas improved melt strength as represented by a decrease in the melt flowrate. A disadvantage of the process disclosed in U.S. Pat. No. 5,554,668is that the production rate of the irradiated polypropylene isrelatively low, because the speed of the conveyor belt is low and only asmall volume of material is processed. This results in difficulties incommercial implementation of the process. In addition, the specificationdiscloses the use of a very broad range of dose rates i.e. from 1 to1×10⁴ Mrads per minute. High dose rates of greater than about 40 Mradcan result in a substantially fully cross-linked structure of thepolypropylene. Such a cross-linked structure is difficult to process.

EP-A-0520773 discloses an expandable polyolefin resin compositionincluding polypropylene optionally blended with polyethylene. In orderto prepare a cross-linked foam, a, sheet of expandable resin compositionis irradiated with ionising radiation to cross-link the resin. Theionising radiation may include electron rays, at a dose of from 1 to 20Mrad. It is disclosed that auxiliary cross-linking agents may beemployed which include a bifunctional monomer, exemplified by1,9-nonanediol dimethyacrylate.

U.S. Pat. No. 2,948,666 and U.S. Pat. No. 5,605,936 disclose processesfor producing irradiated polypropylene. The latter specificationdiscloses the production of a high molecular weight, non-linearpropylene polymer material characterised by high melt strength by highenergy irradiation of a high molecular weight linear propylene polymer.It is disclosed that the ionising radiation for use in the irradiationstep may comprise electrons beamed from an electron generator having anaccelerating potential of 500 to 4000 kV. For a propylene polymermaterial without a polymerised diene content, the dose of ionisingradiation is from 0.5 to 7 Mrad. For propylene polymer material having apolymerised diene content, the dose is from 0.2 to 2 Mrad.

EP-A-0821018 discloses the preparation of cross linkable olefinicpolymers which have been subjected to ionising radiation. Thespecification exemplifies electron beams of relatively low energy andlow doses to split polymeric chains in order to graft silane derivativesonto the polymeric chain. The specification does not address the problemof achieving high melt strength of polymers.

EP-A-0519341 discloses the grafting of vinyl monomers on particulateolefin polymers by irradiating the polymer and treating with a graftingmonomer. In an example, polypropylene is irradiated with an electronbeam having an energy of 2 MeV and subsequently treated with maleicanhydride as a grafting monomer.

U.S. Pat. No. 5,411,994 discloses the production of graft copolymers ofpolyolefins in which a mass of olefin polymer particles is irradiatedand thereafter the mass is treated with a vinyl. monomer in liquid form.The ionising radiation dose is about 1 to 12 Mrad and the ionisingradiation preferably comprises electrons beamed from an electrongenerator having an accelerating potential of 500 to 4000 kV. Thepolymer is first irradiated and then treated with a grafting agent.

EP-A-0792905 discloses the continuous production of polypropylenemixtures of increased stress crack resistance and melt strength by theaction of ionising radiation. The energy of the ionising radiation isfrom 150 to 300 keV and the radiation dose ranges from 0.05 to 12 Mrad.

It is further known that when irradiating isotactic polypropylene whichhas been produced using conventional Ziegler-Natta catalysts, theirradiation of the polypropylene with an electron beam produces freeradicals and there is a competition between chain scission and branchingwhich is in favour of chain scission. It is known to use branchingagents, for example multi-vinylic compounds, to displace the equilibriumtowards the achievement of branching. For example CA-A-2198651 disclosesthat bifunctional, unsaturated monomers can be added before and/orduring the irradiation. Such compounds may include divinyl compounds,alkyl compounds, dienes or mixtures thereof. These bifunctional,unsaturated monomers can be polymerised with the help of free radicalsduring the irradiation. Butadiene is particularly preferred.CA-A-2198651 also discloses a continuous method for producingpolypropylene mixtures of increased stress-crack resistance and meltstrength in which a low-energy electron beam accelerator with an energyof from 150 to 300 keV at a radiation dose of 0.05 to 12 Mrads isemployed. This process also suffers from the disadvantage that theproduction rate of the irradiated powder can be somewhat low forcommercial acceptance. Moreover, the polypropylene powder to beirradiated must be in the form of very fine particles.

The use of such branching (or grafting) agents leads to thedisadvantages of increased cost and increased possibility ofenvironmental problems, in particular toxicity, as a result of addingbranching or grafting agent to the polypropylene.

It is also known to irradiate polypropylene copolymers of propylene anddienes, for example 1,5-hexadiene, after their polymerisation. The useof such copolymer complicates substantially the polymerisationprocedure.

The present invention aims to provide a process for producingpolypropylene resins, having improved properties, in particular improvedmelt strength and higher melt index, and also optionally which can bemanufactured at a high production rate particularly for the manufactureof fibres. It is also an aim of the invention to provide such a processwhich provides substantially increased long chain branching on thepolypropylene molecules following the irradiation, while employingrelatively low irradiation doses. It is a further aim to producepolypropylene having higher melt index.

The present invention provides a process for producing polypropylenehaving increased melt strength, the process comprising irradiatingpolypropylene which has been polymerised using a Ziegler-Natta catalystwith an electron beam having an energy of at least 5 MeV and a radiationdose of at least 10 kGray and mechanically processing a melt of theirradiated polypropylene to form long chain branches on thepolypropylene molecules, whereby the polypropylene has a melt flow index(MFI) of at least 25 dg/min.

The present invention is predicated on the discovery by the presentinventor that high irradiation energy electron beams increase the meltindex of polypropylene homopolymers produced using a Ziegler-Nattacatalyst, without substantially decreasing melt strength. The highenergy electron beams also enable high throughput of polypropylene. Thepolypropylene is irradiated without a branching or grafting agent,because this ensures that branched chains of relatively short length,suitable for the spinning of fibres, are produced. This also makesirradiation more commercially useful and with reduced environmental ortoxicity problems.

Preferably, the polypropylene is irradiated at an energy of at least 10MeV.

The polypropylene may be an isotactic polypropylene, a syndiotacticpolypropylene, or a blend of isotactic and syndiotactic polypropylene.Most particularly, the polypropylene has been polymerised using aZiegler-Natta catalyst, and in particular comprises an isotacticpolypropylene polymerised using a Ziegler-Natta catalyst (hereinafterreferred to as “ZNiPP”). The polypropylene or polypropylene blend mayhave a monomodal molecular weight distribution or a multimodal molecularweight distribution, for example a bimodal molecular weightdistribution. This production of polypropylene with good melt strengthand higher melt index enables the polypropylene to be used in a varietyof different applications where high melt flow coupled with meltstrength is required where the polymer is processed from the melt, forexample in the production of fibres.

The polypropylene may be a homopolymer of propylene or a random or blockcopolymer of propylene and one or more olefins and/or dienes selectedfrom ethylene and C₄ to C₁₀ 1-olefins or dienes, which may be linear orbranched. For example, the polypropylene may be an ethylene-propylenerandom copolymer containing up to 10 wt % ethylene. The polypropylenehomopolymer may be reinforced by rubber particles, for exampleethylene-propylene rubber particles, typically in an amount of up to 30wt %.

In the irradiation process, typically the polypropylene is depositedonto a continuously moving conveyor such as an endless belt. Thepolypropylene on the conveyor passes under an electron beam generatorwhich irradiates the polyolefin polypropylene. Preferably, theaccelerating potential or energy of the electron beam is from 5 to 100MeV, still more preferably at least 10 MeV, yet more preferably from 10to 25 MeV. The power of the electron beam generator is preferably from50 to 500 kW more preferably for 120 to 250 kW. The radiation dose towhich the polypropylene is subjected is preferably from 10 to 100 kGray,preferably around 15 kGray (10 kGray is equivalent to 1 Mrad). Theconveyor speed is adjusted in order to achieve the desired dose.Typically, the conveyor speed is from 0.5 to 20 metres/minute,preferably from 1 to 10 metres/minute, more preferably from 2.25 to 8.5metres/minute.

As a result of the high irradiating potential of the electron beam, notonly can the conveyor speed be significantly higher than in the priorart, but also the thickness of the continuously moving bed ofpolypropylene on the conveyor can be relatively high. Typically, the bedof polypropylene has a thickness of up to 20 cm, most particularly from5 to 10 cm. The bed of polypropylene on the conveyor typically has awidth of up to about 1 metre. Preferably, the irradiation is carried outunder an inert atmosphere, such as nitrogen.

After irradiation by the electron beam, the polypropylene powder can beannealed and then treated with at least one known antioxidant additive.The annealing temperature may range from 50 to 150° C. more preferablyfrom 80 to 120° C. and the annealing time may range from 1 to 60minutes, more preferably from 5 to 30 minutes. Thereafter thepolypropylene is mechanically processed, e.g. by extrusion, andgranulated.

In accordance with a preferred aspect of the invention, the irradiatedpolypropylene has increased melt index coupled with good melt strength.This particular combination of rheological properties provides anoutstanding processing behaviour which allows the polypropylene basedpolymers produced in accordance with the invention to be suitableparticularly-for producing fibres.

The invention will now be described in greater detail with reference tothe following non-limiting examples and the accompanying drawings, inwhich:

FIG. 1 is a graph showing the relationship between melt flow index (MFI)and irradiation dose for polypropylenes produced in accordance withExamples 1 to 5 of the process of the invention and Comparative Example1;

FIG. 2 is a graph showing the molecular distribution of thepolypropylenes of Examples 1 to 3 and Comparative Example 1;

FIG. 3 is a graph showing the relationship between melt strength andirradiation dose for polypropylenes produced in accordance with Examples1 to 5 and Comparative Example 1;

FIG. 4 shows the relationship between melt strength and MFI for thepolypropylenes of Examples 1 to 3 and Comparative Example 1 and alsolinear ZNiPPs having the same molecular characteristics, in particularMFI;

FIG. 5 is a graph showing the relationship between the branching factorg and irradiation dose for the polypropylenes of Examples 1 to 3 andComparative Example 1; and

FIG. 6 is a graph showing the relationship between the activation energyand irradiation dose for the polypropylenes of Examples 1 to 5 andComparative Example 1;

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLE 1

In these Examples and Comparative Example, an isotactic polypropylenewas produced using a Ziegler-Natta catalyst (thereby producing ZNiPPrepresenting Ziegler-Natta-synthesised isotactic polypropylene). Thepolymerisation was performed with the addition of hydrogen gas duringthe polymerisation process. The resultant ZNiPP had a melt flow index(MFI) of around 7 dg/min. In this specification, the melt flow index(MFI) is measured by the procedure of ASTM D 1238 using a load of 2.16kg at a temperature of 230° C. for polypropylene.

The polypropylene was then subjected to electron beam irradiation. Priorto irradiation, the polypropylene fluff was stabilised with 200 ppmIrganox 1076. In particular, the polypropylene was deposited onto anendless belt conveyor having a speed of 2.2 to 8.8 m/minute. Thepolypropylene powder was deposited onto the conveyor as a bed having athickness of 7 cm. The conveyor conveyed the bed under a high energyhigh power electron accelerator. Such accelerators are available incommerce. The accelerator had an energy of 10 MeV and a power of 120 kW.The polypropylene powder was divided into three samples for Examples 1to 3 respectively and irradiated for a period of time (determined by theconveyor speed) sufficient to provide varying respective radiation dosesof 15, 30 and 60 kGray for Examples 1 to 3. During the irradiation, thepowder was maintained under argon (or nitrogen) to exclude oxygen.

After irradiation, the powder was kept under nitrogen and was mixed withconventional antioxidant additives comprising 500 ppm Irganox 3114, 1000ppm Irgafos 168 and 400 ppm calcium stearate.

After the addition of the antioxidant additives, the powder was extrudedand granulated at 220° C. under an argon atmosphere.

The properties of the polypropylene of Examples 1 and 3 and ComparativeExample 1 are shown in Table 1.

The MFI of a sample of the initial non-irradiated ZNiPP of ComparativeExample 1 and the three samples of the ZNiPP of Examples 1 to 3 havingbeen irradiated at the three different radiation doses of 15, 30 and 60kGray were measured and the results are shown in FIG. 1. For ComparativeExample 1 there was no irradiation (i.e. a zero irradiation dose). TheMFI values for radiation doses of 0, 15, 30 and 60 kGray were,respectively, 6.9, 19.1, 51.3 and 210 g/10 min. For the ZNiPP, it may beseen that the irradiation dose significantly increases the MFI, withprogressively increasing MFI for increasing doses. The high MFI valuesrender the irradiated ZNiPP particularly suitable for fibre production.The irradiation dose is selected to achieve the desired properties (e.g.MFI) in the irradiated polymer when starting with a given polymer ofparticular initial MFI.

In addition, for the four samples, namely the unirradiated sample ofComparative Example 1 and the three irradiated samples of Examples 1 to3, the molecular weight distribution was determined by gel phasechromatography and the resultant molecular weight distribution curvesare shown in FIG. 2.

With increasing irradiation dose, the molecular weight distributioncurves are shifted to the lower molecular weight side and the curvesbecome narrower, i.e. with a reduction in the dispersion index D. Thismeans that for ZNiPP, the electron beam irradiation tends to cause morescission of the polymer chains than recombination to form long chainbranched polymers. The kinetics of recombination in long branched chainsis less important that the scission of the chains. This increase in theformation of shorter chains by scission increases the melt flow of thepolymer.

The melt strength of the unirradiated polypropylene of ComparativeExample 1 and the irradiated polypropylene of Examples 1 to 3 at thethree different irradiation doses was then measured at 210° C. The meltstrength was measured using a CEAST rheometer (Rheoscope 1000) equippedwith a capillary die and a rotating wheel as a take-up device. Moltenpolymer was extruded through the capillary drive by application of apressure resulting from the displacement of a piston. The, moltenextrudate was uniaxially stretched before crystallisation by wrappingthe fibre around the rotating wheel. In the test, -the pistondisplacement rate was fixed and the speed of the rotating take-up wheelwas lineally changed at constant acceleration until the fibre, becomingvery thin, breaks. The tensile force was recorded during the test. Thetest was run with a cylindrical die having a length/diameter ratio of 5mm/1 mm. The diameter of the rotating wheel was 120 mm and thedisplacement rate of the piston was 2 mm/min giving an extrudatethroughput of 2.36 mm³/min. The acceleration of the rotating wheel wasconstant at 10 rpm/100 seconds, or 0.000628m/s². The extrusiontemperature was 210° C. During the melt strength experiments, therecorded force rapidly obtained a constant value that remainedindependent of wheel rpm up to rupture.

The melt strength was defined as the maximum tensile force recordedduring the experiment. The melt strength values for each of the fourpolypropylene samples are shown, with respect to the irradiation dose,in Table 1 and FIG. 3.

The relationship between melt strength and irradiation dose is shown inFIG. 3. It will be seen that the melt strength of the ZNiPP decreasesslightly with increasing radiation dose. Since the molecular weight ofthe ZNiPP decreases with increasing irradiation, this causes a decreaseof the melt strength, despite any increase in the presence of long chainbranching which in turn is indicated by an increase in activation energy(Ea) with irradiation.

The melt strength was correspondingly measured at 250° C. both forExamples 1 to 3 and Comparative Example 1. The results are shown inTable 1. The melt strength decreases slightly with increasingirradiation dose.

The melt strength was also measured at 185° C. with a rotating wheeldiameter of 19 cm and with a variable displacement rate of the piston offrom 1 to 12 mm/min. The speed of the rotating wheel was adjusted tokeep constant the titre of the fibre (10 deniers or 11.1 dTex). Thetensile force corresponding to a stretching rate of 3.3 m/s (330 rpm)was defined as the melt strength.

The melt strength of the polypropylenes of Examples 1 to 3 andComparative Example 1 was measured at 185° C. and the results are shownin Table 1 and FIG. 4. The melt strength for Examples 1 to 3 is the sameas or lower than that for Comparative Example 1. In addition, the meltstrength for a corresponding number of samples of linear (nonirradiated) ZNiPP having varying MFI values substantially the same asthose of Examples 1 to 3 and Comparative Example 1 was measured and theresults are shown in FIG. 4. At substantially the same MFI values forthe linear (non-irradiated) ZNiPP and the irradiated ZNiPP (the same MFIvalues indicating substantially the same molecular characteristics) themelt strength of the irradiated ZNiPP is higher than that ofnon-irradiated ZNiPP.

For linear non-irradiated ZNiPP, the melt strength is significantlylower than that of the irradiated ZNiPP of similar MFI and decreasessignificantly with increasing MFI.

At MFI values greater than about 25 dg/min (achievable for theparticular starting MFI of 6.9 dg/min at irradiation doses above about15 kGray) it may be seen that the melt strength of the irradiated ZNiPPis significantly higher than that of the non-irradiated ZNiPP ofcorresponding MFI. Thus for such irradiated ZNiPP having an MFI greaterthan around 25 dg/min, when formed into thin fibres at high fibrespinning speeds yet at relatively low temperatures of around 185° C., ahigh degree of melt strength is obtained. Thus the irradiated ZNiPP canbe spun readily and reliably. The use of irradiation for ZNiPP offsets arapid decrease in melt strength with increase in MFI, corresponding to adecrease in molecular weight. For the linear non irradiated ZNiPPsamples employed in FIG. 4, hydrogen was added to the reactors toachieve a high MFI for the high MFI samples.

It is known that a strong increase of melt viscosity (η) at lowfrequency is observed for polypropylene containing long chain branchingstructures. The relationship between the shear viscosity of thepolypropylene melt as a function of circular frequency is dependent uponthe degree of long chain branching.

In order to quantify the amount of long chain branching in isotacticpolypropylene, the applicant has formulated a parameter, referred toherein as the branching factor g, which is determined from theTheological properties of the polymer. The long chain branching factor gwas established by the ratio Mw (COP)/Mw (η0) where Mw (COP) is theweight average molecular weight at the crossover point coordinates(W_(c) and G_(c)) (as discussed hereinbelow) and Mw (η0) is the weightaverage molecular weight at zero shear viscosity. The branching factoris equal to 1±0.05 for linear isotactic polypropylene and is less than 1for isotactic polypropylene with long chain branching. The branchingfactor g is determined from the ratio of two weight average molecularweight (M_(w)) values inferred from a dynamic frequency sweep on aviscoelastimeter such as the models available in commerce under thetrade names RDA 700 or ARES 700 from the company RheometricsScientifics.

The branching factor is determined as follows. For the viscoelastimeteravailable from Rheometrics Scientifics under the trade name ARES, theoperating conditions were set up as follows: the strain was set up to befrom 10 to 20%, i.e. in the linear viscoelastic range; the frequencysweep was from 0.1 to 500 radians/second; the plate-plate geometry was25 mm diameter, with a polymer thickness therebetween of typicallyaround 2 mm. In some instances, the same testing experiment wasconducted at different melt temperatures, for example at 190° C. and210° C., and the viscoelastic responses expressed at the referencetemperature of 230° C. using a master curve approach, which is describedin the paper entitled “Temperature—dependence of polyolefin meltrheology”, H. Mavridis and R. N. Shroff, Polymer Eng. Sci. 32, 1778(1992).

From the data obtained, the storage (G′) and loss (G″) shear moduli, aswell as the complex shear melt viscosity (η*) were plotted as a functionof circular frequency at the reference temperature of 230° C. Across-over point (COP) for the storage and loss shear moduli wasobserved for all the isotactic polypropylenes investigated. Thecross-over point (COP) coordinates G′=G″=G_(c) and the correspondingcircular frequency W_(c) can be used to infer information pertaining tothe weight average molecular weight M_(w) and its polydispersity asfirst proposed in the paper by G. R. Zeichner and P. D. Patel, Proc.2^(nd) World Cong. Chem. Eng. 6, 333 (1981).

The applicants tested 33 linear isotactic polypropylenes with M_(w)values ranging from 70 kDa to 1200 kDa and polydispersity index(D=M_(w)/M_(n)) values of from 2 to 25 and found the following equationfor the molecular weight at the cross-over point to apply:M _(w)(COP)=exp (6.767−0.187*(LnWc)−0.0129*(LnWc)²)

The weight average molecular weight (Mw) is specified in kDa, and iscalculated with a standard deviation estimated to be around 5%.

For the value M_(w) (η0) which is the weight average molecular weight atzero shear viscosity, this is calculated as follows. From the shearviscosity curve, it is possible to extrapolate the viscosity to the zeroshear rate viscosity using an equation known as the Carreau-Yasudaequation which is described in the paper entitled “Correlation BetweenMolecular Structure and Rheological Behaviour of Polypropylene”, K.Bernreitner, W. Neissl and M. Gahleitner, Polymer Testing, 11, 89(1992). As is well known in the literature, a power law relationshipexists between η₀ and M_(w). Accordingly, using the same data as set outfor the cross-over point, the following equation for the weight averagemolecular weight at zero shear viscosity has been determined:M _(w)(η₀)=exp (3.5897+0.267*Ln(η₀)).

The weight average molecular weight M_(w) is expressed in kDa with astandard deviation around 6%. The viscosity is expressed in Pascal.seconds.

The branching factor g for any given isotactic polypropylene is theratio between the calculated value and M_(w) (COP)/M_(w)(η₀).

For the four samples of Examples 1 to 3 and Comparative Example 1, therelationship between the branching factor and the irradiation dose isshown in FIG. 5. The branching factor for the non-irradiated ZNiPPsample of Comparative Example 1 was around 1, indicating linearity. Itmay be seen that the degree of long chain branch increases, asrepresented by decreasing branching factor g, with increasingirradiation dose.

FIG. 6 shows the relationship between activation energy and irradiationdose for Comparative Example 1 and Examples 1 to 3. The activationenergy represents the energy necessary to activate the molecule to movein the melt. It may be seen that with increasing irradiation dose, theactivation energy increases. This indicates that long chain branchingincreases with increasing irradiation dose, since this is manifested inthe increased activation energy.

For the ZNiPP, a lot of chains were cut by the irradiation, therebyincreasing the MFI.

COMPARATIVE EXAMPLES 2 AND 3

For Comparative Examples 2 and 3, linear polypropylenes having abranching factor of 1.0 and MFI values substantially corresponding tothe MFI values of Examples 2 and 3 respectively, were tested todetermine their molecular weight distributions, melt strength andactivation energy and the results are shown in Table 1. It will be seenfrom a comparison of Comparative Example 2 and Example 2, and from acorresponding comparison of Comparative Example 3 and Example 3, thatfor linear polypropylenes of substantially the same melt flow propertiesas an irradiated polypropylene, the melt strength is significantly lowerat 185° C. and the activation energy is also significantly lower. It maythus be seen that at equivalent melt flow index when comparingirradiated and non-irradiated polypropylenes, the melt strength and theactivation energy are both increased by the irradiation.

The polypropylenes of Examples 1 to 3 were formed into fibres and thetenacity, strain at break and toughness of the fibres were determinedfor different stretching ratios of the fibres and the results are shownin Table 2. The fibres exhibited good mechanical properties.

COMPARATIVE EXAMPLES 4 TO 6

Linear polypropylenes having substantially the same melt flow index asExamples 1 to 3 for, respectively, Comparative Examples 4 to 6, wereformed into fibres and subjected to the same stretching ratios as thefibres of Examples 1 to 3. The results are shown in Table 3. From acomparison of Tables 1 and 3, it may be seen that mechanical propertiesof the fibres of Examples 1 to 3 are substantially the same or onlyslightly lower than those of the linear molecules of ComparativeExamples 4 to 6. However, as a result of the irradiation process, themelt strength of the polypropylenes of Examples 1 to 3 will have beensignificantly increased, thereby greatly increasing the processabilityof the polypropylenes of Examples 1 to 3, particularly when used for theproduction of fibres.

In Tables 2 and 3, the fibres which were tested were spun on a, Lablinespinning machine at a melt temperature of 250° C. with a 40-holespinneret at a melt throughput of 290 grams per hour. Fibres of 10-dTex(10 grams per 10,000 meters) were produced with nominal stretch ratios(SR) being a ratio of fast to slow godets of from 2 to 4.

Good mechanical properties were obtained with the irradiated samples.The tenacity is slightly decreased as a result of the irradiation, thestrain at break increases and the toughness, which is a combination ofboth factors, is a little bit lower or equivalent at high melt flowindexes.

EXAMPLE 4

For Example 4 the ZNiPP of Example 3 was treated at the same irradiationdose of 60 kGray but the antioxidant additives were 700 ppm Irganox 1010and 1100 ppm Irgafos 168. The molecular weight distribution, the meltstrength and the activation energy were determined for the resultantpolymer and the results are shown in Table 1.

The use of an irradiation dose of 60 kGray greatly enhances the MFI ofthe polymer but to a lesser degree than for Example 3, yet maintains ahigher melt strength than for Example 3, the melt strength beingsubstantially the same at 185° C. and 250° C. as for the unirradiatedpolymer. The activation energy of Examples 3 and 4 is about the same,indicating a similar degree of long chain branching. TABLE 1 ZNiPP Comp.Comp. Comp. Polymer Example 1 Example 1 Example 2 Example 3 Example 2Example 3 Example 4 Irradiation 0 15 30 60 0 0 60 dose(kGray) MFI(dg/min) 6.9 19.1 51.3 210 51.3 190 93 Mn (kDa) 48.2 45.8 37 29.5 27.122.1 33.2 Mw (kDa) 297 211 176 122 171 108 167 Mz (kDa) 1347 687 628 423764 379 654 D 6.2 4.6 4.8 4.1 6.34 4.9 5 D′ 4.5 3.2 3.6 3.5 4.5 3.5 3.9Branching 0.97 0.81 0.72 0.6 1.0 1.0 — factor g Melt Strength 8.6 8.68.3 3.3 3.2 0 8 @ 185° C. (mN) Melt Strength 5 4.5 3.5 1.5 — — 4 @ 210°C. (mN) Melt Strength 3.5 2.5 2 0 — — 2 @ 250° C. (mN) Ea (kJ/mole) 40.763.6 71.3 67.6 39 38 67.6

TABLE 2 irradiated molecules Example 1 Example 2 Example 3 MFI (dg/min)19.1 51.3 210 Stretching Stretching Stretching ratio ratio ratio 2 3 22.5 2 3 4 Tenacity T 2.4 3.3 1.8 1.7 1.5 1.7 1.8 (cN/dTex) Strain at 21092 232 268 319 207 95 break E(%) Toughness = 35.1 31.6 26.9 27.0 26.625.0 17.2 T√E

TABLE 3 linear molecules Comp. Example Comp. Example Comp. Example 4 5 6MFI (dg/min) 21 48 201 Stretching Stretching Stretching ratio ratioratio 2 3 2 2.5 2 3 4 Tenacity T 3.8 4.9 2.4 2.4 1.8 2.5 3.1 (cN/dTex)Strain at 138 81 180 187 219 111 71 break E(%) Toughness = 44.8 44.032.1 27.0 26.0 26.7 26.3 T√E

1. A process for producing a propylene polymer having increased meltstrength, the process comprising irradiating a propylene polymer whichhas been polymerised using a Ziegler-Natta catalyst with an electronbeam having an energy of at least 5 MeV and a radiation dose of at least10 kGray and mechanically processing a melt of the irradiated propylenepolymer to form long chain branches on the propylene polymer molecules,whereby the propylene polymer has a melt flow index (MFI) of at least 25dg/min.
 2. A process according to claim 1 wherein the electron beam hasan energy of at least 10 MeV.
 3. A process according to claim 1 whereinthe electron beam has an energy within the range of 10-25 MeV.
 4. Aprocess according to claim 1 wherein the power of the electron beam isfrom 50 to 500 kW.
 5. A process according to claim 1 wherein the powerof the electron beam is from 120-250 kW.
 6. A process according to claim1 wherein the radiation dose is within the range of 10-60 kGray.
 7. Aprocess according to claim 1 wherein said propylene polymer is apolypropylene homopolymer.
 8. A method of claim 1 wherein said propylenepolymer is a copolymer of propylene and at least one alpha olefinselected from a group consisting of ethylene and C₄-C₁₀ 1-olefins.
 9. Aprocess accorded to claim 1 wherein said propylene polymer is isotacticpolypropylene.